Infrared imaging system

ABSTRACT

An infrared imaging system is disclosed which provides a two-dimensional visible image of the temperature profile of a radiating infrared source remotely located from the system. The invention is preferably implemented as a small hand-held instrument which can be held at a user&#39;s eye for sighting at a source of radiation and for viewing the temperature profile thereof. The system comprises an objective lens operative to focus an infrared image of a radiating source onto a surface of a liquid crystal structure, with the opposite surface of the structure being visually viewable by way of an eyepiece. The liquid crystal structure is biased preferably by a closed loop temperature control to be sensitive to an intended infrared spectrum of interest and to be substantially non-responsive to ambient temperatures.

FIELD OF THE INVENTION

This invention relates to infrared detection systems and moreparticularly to a passive infrared imaging system for sensing theinfrared radiation provided by a source and providing a visible image ofthe sensed radiation.

BACKGROUND OF THE INVENTION

Infrared detection systems are known for sensing infrared radiation froma radiating object or source and for providing an indication of sensedtemperature or a display of temperature profile. Many infrared detectioninstruments are in the nature of non-contacting thermometers forproviding measurement of radiation from a single localized area at whichthe instrument is aimed. Such instruments generally employ an infrareddetecting element which provides an electrical signal in response toreceived radiation and which signal is processed by associatedelectronic circuitry to provide an output indication of sensedtemperature. Systems are also known in which electronic or mechanicalscanners are employed to produce a two-dimensional display of sensedradiation and which display is provided typically on a cathode ray tubescreen or on a pen recorder. These systems also require activeelectronic circuitry and an active sensing instrumentality such as animage tube or a sensor element and associated scanner. As a consequence,such known systems are relatively expensive and complex.

Liquid crystal materials are also known for providing a visualrepresentation of temperature in the form of colors which correspond tosensed temperatures. In general, such liquid crystal materials areapplied either in film form or as a coating to a radiating surface fordirect sensing of surface temperature and to exhibit by virtue of colorchanges the surface temperature profile.

SUMMARY OF THE INVENTION

In accordance with this invention, an infrared imaging system isprovided in which a two-dimensional visible image is provided of aradiating source remotely located from the system. The invention ispreferably implemented as a hand-held instrument which can be held ateye level for sighting of a radiating source and viewing of thetemperature profile thereof. The system includes an objective lensoperative to focus an infrared image of a radiating source or objectonto a surface of a liquid crystal structure, with the opposite surfaceof the structure being visually viewable by way of an eyepiece lens. Theliquid crystal provides varying colors corresponding to the temperaturethat is reached by various portions of the crystal material in responseto received radiation from an infrared source. The visually perceivablecolor pattern is representative to the temperature profile of theinfrared source. Such a profile is provided without any contact with theradiating object, and by passive apparatus which is relatively simple toconstruct and to operate.

The liquid crystal material is biased by a heater to a temperature abovethe ambient temperature and just below the temperature at which itscolor changes from black to red. The liquid crystal thus respondsprimarily to the infrared radiation impinging on it and not to ambienttemperature. Preferably the liquid crystal is part of a laminatedstructure or sandwich which includes a transparent electrical heaterfilm over the liquid crystal layer, and facing the eyepiece lens. Anabsorbtive backing is provided on the other surface of the liquidcrystal layer to absorb radiant energy imaged thereon by the objectivelens. The heater can be controlled in a closed servo loop in which poweris applied to the heater of a magnitude sufficient to maintain thetemperature of the liquid crystal sheet at its intended transition stateas sensed by light reflected from the liquid crystal surface and fromwhich a heater control signal is derived. In an alternative embodiment,a visual image of the object being observed can be superimposed upon theinfrared image viewed on the liquid crystal sheet.

DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a diagrammatic representation of an infrared imaging systemembodying the invention;

FIG. 2 is a diagrammatic representation of the liquid crystal sheetstructure and associated biasing system employed in the invention; and

FIG. 3 is a diagrammatic representation of an alternative embodiment ofthe invention providing a superimposed visual image.

DETAILED DESCRIPTION OF THE INVENTION

The invention as shown in preferred embodiment in FIG. 1 comprises ahousing 10 having a liquid crystal structure 12 mounted therein, thisstructure being of laminated construction as will be described below. Anobjective lens 14 is disposed at one end of the housing 10, and aneyepiece lens 16 is disposed at the other end of the housing. Theobjective lens is transmissive to and operative to focus infraredradiation in an intended spectral range, typically at wavelengths of 0.7to 10 microns. Preferably the objective lens is a Fresnal lens molded orotherwise formed in a relatively thin plastic sheet. Such Fresnel lensesare per se well known and are of considerably less cost than the morevisual lenses formed of optical glass or plastic having multicurvedsurfaces. The eyepiece lens is operative for visual light and may becomposed of one or more lens elements, in well-known manner, to provideintended optical magnification. The object lens 14 has a focal lengthdetermined to provide imaging of infrared radiation from an object beingviewed onto the surface 18 of the liquid crystal structure. The eyepiecelens is of a focal length to provide intended magnification of the imageproduced by the liquid crystal structure as viewed through the eyepieceby a user.

A light source 20 and light detector 22 are provided within housing 10and are part of automatic heater control apparatus 24 for biasing of theliquid crystal material to an intended operating state. The light sourceis operative to illuminate a portion of the confronting liquid crystalsurface, while the light sensor is operative to sense light reflectedfrom the illuminated liquid crystal surface and to provide an electricalsignal to the heater control which is operative in a manner to bedescribed.

The liquid crystal structure is shown in greater detail in FIG. 2 and iscomposed of a centrally disposed sheet 30 of liquid crystal materialsandwiched between a transparent electrical heater layer 32 whichconfronts the eyepiece of the system, and an infrared radiationabsorbing layer 34 which confronts the objective lens of the system. Theheater control 24 provides energization of the light source 20 whichilluminates a portion of the liquid crystal surface, and also provides acontrol signal to a heater power source 36 which provides energizingpower to the heater layer 32 of a magnitude determined by the controlsignal. Light reflected from the liquid crystal surface at the point atwhich its color starts to change from black is detected by the lightsensor 22 which provides a feedback signal to the control electronicsfor maintaining closed loop temperature control. The liquid crystalmaterial enclosed is sensitive to temperatures above the normal ambienttemperature, and is biased to a temperature level above the ambienttemperature range and just below the point where its color changes fromblack to red. Power is applied to the heater layer 32 to raise thetemperature thereof and the laminated liquid crystal sheet to anintended temperature level as determined by a manual control 38, or asmay be preset in the heater control 24. Thus, the liquid crystalmaterial is biased to respond substantially only to the impinginginfrared radiation and not to ambient temperature.

The heater layer 32 can be constructed from electrically conductingglass cemented to the surface of the liquid crystal sheet 30. The liquidcrystal material is an organic material known as a cholesteric esterwhich is bound in a Mylar or other plastic substrate. The layer 34 istypically a black paint on the surface of sheet 30 opposite heater layer32, to provide enchanced infrared absorbtion.

In operation, a user holds the apparatus at eye level for viewingthrough the eyepiece, sights the apparatus at an object or source to bemonitored and observes on the viewing surface of the liquid crystalstructure a pattern of colors and hues representative of the temperatureprofile of the infrared radiation being received. It is evident thatcontact with the radiating source is not necessary, and the apparatusembodying the invention can be employed over varying distances from thesource of radiation. With presently available liquid crystal materials,the invention is operable typically up to distances of about 50 feet.

An alternative embodiment is shown in FIG. 3 and by which a visual imageis superimposed on the radiation image provided by the liquid crystalstructure. The housing 10a includes the liquid crystal structure,objective lens, and eyepiece lens as in the embodiment described above,and in addition includes a half silvered mirror 50 disposed at a 45°angle as illustrated between the eyepiece lens and the liquid crystalstructure. A mirror 52 is disposed in an upper portion of the housingalso at a 45° orientation as illustrated. An objective lens 54 isprovided at the forward end of the upper housing portion and operativeto refract visual light. The optical channel is constructed to provide avisual image of the object being viewed of substantially the same sizeas the radiation image viewed on the liquid crystal sheet and insubstantial superimposition therewith, by providing focal lengths andinternal optical path lengths for both objectives which are the same. Inemploying this embodiment, a user will see a visible image of an objectbeing viewed, with a variably colored pattern superimposed on the viewedobject representative of the temperature profile of the object. Thesuperimposed optical image can be eliminated selectively by occlusion ofthe optical path, such as by moving the mirror 52 to a secondaryposition, shown in dotted form in FIG. 3, or by covering the objectivelens 54. As a variation of the embodiment of FIG. 3, a translucentscreen 58 can be provided between the mirror 52 and the objective lens54 and onto which the optical image of the object being viewed isfocused. The optical image on the translucent screen is viewed by meansof the eyepiece lens and associated mirrors. The position of thetranslucent screen and the optical path are again determined to providea superimposed image of substantially equal size as the radiation imageprovided by the liquid crystal sheet.

The invention is not to be limited by what has been particularly shownand described except as illustrated in the appended claims.

What is claimed is:
 1. An infrared imaging system comprising:a liquid crystal structure having a surface for absorbing infrared radiation from a radiating object, and a viewing surface for providing an image representative of the temperature profile of received infrared radiation; means for biasing the liquid crystal material to a temperature level above the ambient temperature range and just below the temperature at which the color of the liquid crystal material changes from black; an objective lens disposed in spaced relationship with the radiation absorbing surface of said structure and operative to image infrared radiation from an object being viewed onto said absorbing surface; an eyepiece lens disposed in spaced relationship with the viewing surface of said structure and operative to provide to a user a magnified version of the image provided thereon; and a visual channel providing a visual image of an object being viewed in superposition with the infrared temperature profile thereof, said visual channel including an objective lens operative to provide a visual image of the object being viewed and means for coupling the visual image to said eyepiece lens for the viewing of the visual image in superposition with the infrared temperature profile thereof.
 2. The system of claim 1 wherein said biasing means includes:a transparent electrical heater film disposed over the liquid crystal structure and operative to heat the liquid crystal material to said temperature level above the ambient temperature range and below the temperature at which the color of the liquid crystal material changes from black.
 3. The system of claim 2 wherein said biasing means includes a light source operative to illuminate a portion of the viewing surface;a light detector operative to provide an electrical signal representative of light reflected from the viewing surface; and a control circuit operative in response to the signal from the light sensor to provide a power signal to the heater film to maintain the liquid crystal material at the intended temperature level.
 4. The system of claim 1 wherein said liquid crystal structure is operative to provide as an image a color pattern representative of the temperature profile of received infrared radiation.
 5. The system of claim 1 wherein said objective lens is a Fresnel lens.
 6. The system of claim 1 wherein said liquid crystal structure includes an infrared radiation absorbing layer confronting received infrared radiation.
 7. The system of claim 1 including a housing having said objective lens on one end thereof, said eyepiece lens on the other end thereof, and wherein said liquid crystal structure and biasing means are disposed within the housing between the objective and eyepiece lenses.
 8. An infrared imaging system comprising:a liquid crystal structure having a surface for absorbing infrared radiation from a radiating object, and a viewing surface for providing an image representative of the temperature profile of received infrared radiation, said liquid crystal structure including a multi-layer structure including a sheet of liquid crystal material, a transparent electrical heater layer confronting the eyepiece lens, and an infrared radiation absorbing layer adapted to confront an objective lens; means including said electrical heating layer for biasing the liquid crystal material to a temperature level above the ambient temperature range and just below the temperature at which the color of the liquid crystal material changes from black; an objective lens disposed in spaced relationship with the radiation absorbing layer and operative to image infrared radiation from an object being viewed onto said absorbing layer; and an eyepiece lens disposed in spaced relationship with the viewing surface of said structure and operative to provide to a user, a magnified version of the image provided thereon.
 9. An infrared imaging system comprising:a liquid crystal structure having a surface for absorbing infrared radiation from a radiating object, and a viewing surface for providing an image representative of the temperature profile of received infrared radiation; means for biasing the liquid crystal material to a temperature level above the ambient temperature range and just below the temperature at which the color of the liquid crystal material changes from black; an objective lens disposed in spaced relationship with the radiation absorbing surface of said structure and operative to image infrared radiation from an object being viewed onto said absorbing surface; an eyepiece lens disposed in spaced relationship with the viewing surface of said structure and operative to provide to a user, a magnified version of the image provided thereon; and a visual channel providing a visual image of an object being viewed in superposition with the infrared temperature profile thereof, said visual channel including an objective lens operative to provide a visual image of the object being viewed and means for coupling the visual image to said eyepiece lens for the viewing of the visual image in superposition with the infrared temperature profile thereof, said visual channel including mirror means for coupling the visual image to said eyepiece lens for viewing of the visual image in superimposition with the temperature profile of said object.
 10. The system of claim 9 wherein said mirror means includes a half-silvered mirror disposed between the eyepiece lens and the liquid crystal structure and operative to permit viewing therethrough of the infrared image of the viewing surface of the liquid crystal structure and to reflect the visual image to the eyepiece lens.
 11. The system of claim 9 wherein said visual channel includes means for occluding the visual channel to selectively eliminate the visual image.
 12. An infrared imaging system comprising:a liquid crystal structure having a surface for absorbing infrared radiation from a radiating object, and a viewing surface for providing an image representative of the temperature profile of received infrared radiation; means for biasing the liquid crystal material to a temperature level above the ambient temperature range and just below the temperature at which the color of the liquid crystal material changes from black; an objective lens disposed in spaced relationship with the radiation absorbing surface of said structure and operative to image infrared radiation from an object being viewed onto said absorbing surface; an eyepiece lens disposed in spaced relationship with the viewing surface of said structure and operative to provide to a user, a magnified version of the image provided thereon; and a visual channel providing a visual image of an object being viewed in superposition with the infrared temperature profile thereof, said visual channel including an objective lens operative to provide a visual image of the object being viewed and means for coupling the visual image to said eyepiece lens for the viewing of the visual image in superposition with the infrared temperature profile thereof, said visual channel including a translucent screen onto which the visual image is formed by the visual channel objective lens; and mirror means for coupling the visual image from the translucent screen to said eyepiece lens for viewing of the visual image in superimposition with the temperature profile of said object. 